The spark plug for use in internal combustion engine to be employed for igniting the internal combustion engine such as an automotive engine is generally provided with: a cylindrical metal shell; a cylindrical insulator provided in the inner hole of the metal shell; a center electrode provided in the leading end side inner hole of the insulator; and a ground electrode having one end bonded to the leading end side of the metal shell and having another end face forming a spark discharge gap together with the center electrode.
As the electrode material to be used as the center electrode and the ground electrode of the spark plug for use in internal combustion engine, there has been known an alloy group, which is called the M-CrAlY, for example. Here, M is a composite material which is composed of Ni (nickel), Co (cobalt) or Fe (iron), or a composite of Ni, Co and Fe such as NiCo or FeCo, and which contains Cr (chromium) in 15 to 30 wt. %, Al (aluminum) in 5 to 15 wt. %, and Y (yttrium) in about 0 to 2 wt. % (as referred to JP-A-63-138681, for example).
There are also known: a Ni-group alloy (as referred to JP-A-64-87738, for example), in which 0.5 to 1.5 wt. % of Si, 0.7 to 2.8 wt. % of Mn, and 0.25 to 4.5 wt. % of Al are added to Ni; a Ni-group alloy (as referred to JP-A-4-45239, for example), in which 1.0 to 2.5 wt. % of Si, 0.5 to 2.5 wt. % of Cr, 0.5 to 2.0 wt. % of Mn, and 0.6 to 2.0 wt. % of Al are added to Ni; and a Ni-group alloy (as referred to JP-A-2004-11024, for example), in which 1.8 to 2.2 wt. % of Si, 0.05 to 0.1 wt. % of one or more kinds selected from Y, Hf and Zr, and 2 to 2.4 wt. % of Al are added to Ni. These individual components in the electrode material of the spark plug for use in internal combustion engine are added to improve the sulfur-resistance, corrosion resistance to lead, and high-temperature oxidation resistance and to suppress the electrode decrease by the spark discharge thereby to improve durability.
In recent years, the purification of fuels has advanced considering the influences on the environment to reduce the sulfur components and the lead components in the fuels so that the demands of the sulfur resistance and the lead resistance for the electrode of the spark plug for use in internal combustion engine have become less than those of the prior art. On the other hand, the more suppression of the decrease of the electrode of the spark plug for use in internal combustion engine by the spark discharge is desired from the viewpoint to improve the durability.
As the spark plug for use in internal combustion engine stressing the suppression of the decrease of the electrode by the spark discharge more than the improvements in the sulfur resistance and the resistance to the lead corrosion, therefore, there is known the spark plug for use in internal combustion engine (as referred to JP-A-2004-206892, for example) using an electrode material, which contains Si in 0.5 to 1.5 wt. %, Al in 0.5 to 1.5 wt. %, at least one of Y, Nd and Sm in 0.05 to 0.5 wt. %, and Cr and Mn in 0.8 wt. % or less in total, and the remainder being Ni and an unavoidable impurity, and which has a specific resistance of 25 μΩcm or less at the room temperature (at about 20° C.).